The failure of seasonal adaptations can have a critical impact on an organism's survival and reproductive success. In people, one example of this is Seasonal Affective Disorder (SAD). Although the mechanisms of SAD remain elusive, its cause is seasonal variation in light conditions. In order to integrate information about seasonal change into their development, many organisms have evolved mechanisms to sense changes in day length. The long-term goal of our research program is to elucidate the molecular mechanisms of day- length measurement. Although the molecular mechanisms of photoperiodism have not yet been well described in many organisms, recent advances in the study of photoperiodic flowering in the model plant Arabidopsis have shed some light on them. In Arabidopsis, the core of photoperiodic time-measurement mechanisms is circadian regulated transcription of the floral activator CONSTANS (CO) gene, and light regulated CO protein stability and activity. In this proposal, we intend to further characterize these core molecular mechanisms by focusing on the functions of FKF1 E3 ubiquitin ligase through genetic, biochemical, and genomic approaches. The knowledge obtained in plant photoperiodism has often facilitated discoveries in other systems. Therefore, findings made in this proposal will be important not only for plant research, but also for broader understanding of photoperiodism in mammals and other systems. In addition, the findings may even provide some conceptional cues to understanding the mechanisms of SAD. Moreover, this type of transcriptional regulation is likely conserved in all eukaryotes, thus the findings will contribute to the understanding of a fundamental transcriptional regulation mechanism. In conclusion, elucidating the photoperiodic flowering mechanism is important not only for understanding a major plant reproduction mechanism, one that is directly applicable for an improvement of crop yield (an important contributor to human health, especially in developing countries) but also in understanding a sophisticated ubiquitination-dependent transcriptional mechanism that is universal among organisms.